Electrochemical fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst typically induces the desired electrochemical reactions at the electrodes. In addition to electrocatalyst, the electrodes may also comprise a porous electrically conductive sheet material, or "electrode substrate", upon which the electrocatalyst is deposited. The electrocatalyst may be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon.
Solid polymer electrolyte fuel cells employ a membrane electrode assembly ("MEA") which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. Separator plates, or flow field plates for directing the reactants across one surface of each electrode substrate, are disposed on each side of the MEA. Solid polymer fuel cells operate at relatively low temperatures (circa 80.degree. C.) compared to other fuel cell types.
A broad range of reactants can be used in electrochemical fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
The fuel stream may contain impurities that do not contribute to, and may actually inhibit, the desired electrochemical reaction. These impurities may, for example, originate from the fuel stream supply itself, or may be generated, for example, as intermediate species during the fuel cell reactions, or may be impurities entering the fuel stream from elsewhere in the system. Some of these impurities may be chemically adsorbed or physically deposited on the surface of the anode electrocatalyst, blocking the active electrocatalyst sites and preventing these portions of the anode electrocatalyst from inducing the desired electrochemical fuel oxidation reaction. Such impurities are known as electrocatalyst "poisons" and their effect on electrochemical fuel cells is known as "electrocatalyst poisoning". Electrocatalyst poisoning thus results in reduced fuel cell performance, where fuel cell performance is defined as the voltage output from the cell for a given current density. Higher performance is associated with higher voltage for a given current density or higher current for a given voltage.
In the absence of countermeasures, the adsorption or deposition of electrocatalyst poisons may be cumulative, so even minute concentrations of poisons in a fuel stream, may, over time, result in a degree of electrocatalyst poisoning which is detrimental to fuel cell performance.
Reformate streams derived from hydrocarbons or oxygenated hydrocarbons typically contain a high concentration of hydrogen fuel, but typically also contain electrocatalyst poisons such as carbon monoxide. To reduce the effects of anode electrocatalyst poisoning, it is known to pre-treat the fuel supply stream prior to directing it to the fuel cell. For example, pre-treatment methods may employ catalytic or other means to convert carbon monoxide to carbon dioxide. However, known pretreatment means for reformate streams cannot efficiently remove 100% of the carbon monoxide. Even trace amounts less than 10 ppm can eventually result in electrocatalyst poisoning which causes a reduction in fuel cell performance.
Substances other than carbon monoxide are also known to poison fuel cell electrocatalysts. Poisons may also be generated by the reaction of substances in the reactant streams with the fuel cell component materials. For instance, carbon monoxide or other impurities can be generated from carbon dioxide in the presence of an electrocatalyst. This can occur when there is a relative abundance of carbon dioxide and a relatively low concentration of carbon monoxide such that equilibrium conditions favor some limited carbon monoxide formation. What constitutes a poison may depend on the nature of the fuel cell. For example, whereas methanol is the fuel in a direct methanol fuel cell, in a hydrogen fuel cell operating on a methanol reformate stream, traces of unreformed methanol can be detrimental to the electrocatalyst performance.
Conventional methods for addressing the problem of anode electrocatalyst poisoning include purging the anode with an inert gas such as nitrogen. However, such purging methods involve suspending the generation of power by the fuel cell, thus a secondary power source may be needed to provide power while the fuel cell anode is being purged.
Another approach for removing poisons from an electrocatalyst comprises introducing a "clean" fuel stream containing substantially no carbon monoxide or other poisons to a poisoned fuel cell anode. Where the adsorption is reversible, an equilibrium process may result in some rejuvenation of the electrocatalyst. However, a disadvantage of this approach is that it is generally not effective against irreversibly adsorbed poisons. Furthermore, the recovery of the anode electrocatalyst by such an equilibrium process can be very slow, during which time the fuel cell is not able to operate at full capacity.
Another technique to counteract carbon monoxide electrocatalyst poisoning is to continuously introduce a low concentration of oxygen into the fuel stream upstream of the fuel cell, as disclosed in U.S. Pat. No. 4,910,099. Therein, oxygen levels from about 2% to 6% were injected into fuel streams having carbon monoxide levels from about 100 to 500 ppm. However, such an oxygen bleed into the fuel stream results in some consumption of hydrogen and hence a reduction in fuel efficiency. Further, an oxygen bleed results in undesirable localized exothermic reactions at the anode, particularly near the fuel inlet, which may damage fuel cell membranes and reduce fuel cell lifetime. Further still, since in practice oxygen bleed is typically obtained by compressing air (it is often drawn from a compressed air stream provided as a fuel cell oxidant), use of an oxygen bleed may result in an additional parasitic load on a fuel cell system. Thus, the use of an excessive amount of oxygen bleed is undesirable.
To efficiently and effectively counteract carbon monoxide poisoning, it is desirable to know the approximate concentration of carbon monoxide in the fuel stream. However, directly measuring the concentration of carbon monoxide in a fuel stream can be difficult in practice. Thus, while a sensor for directly measuring carbon monoxide is desirable in a fuel cell system, often the concentration is inferred based on the known operating conditions of the fuel cell system (e.g., carbon monoxide concentrations can be determined under various operating conditions in a laboratory and can then be inferred for a fuel cell system operating under similar conditions in actual use). While many early fuel cell applications may have had relatively constant operating conditions and therefore relatively consistent levels of carbon monoxide in the fuel stream, the potential applications for fuel cells are expanding. As a result, the carbon monoxide concentration in the fuel streams now tends to vary for many reasons (e.g., depending on reformer temperature, or fuel cell and/or reformer load demand conditions).
A low output voltage from one or more of the fuel cells in a fuel cell system might be used as an indicator of carbon monoxide poisoning. Preferably perhaps, a sensor cell might be incorporated in the fuel cell system for this purpose. As described in U.S. Provisional Patent Application Ser. No. 60/091,531 filed Jul. 2, 1998, by the same applicant as the present application and previously incorporated herein by reference in its entirety, a sensor cell can be incorporated whose performance is more sensitive to carbon monoxide poisoning than other fuel cells in the fuel cell system. Thus, a sensor cell can be used to indicate an abnormal or undesirable operating condition (e.g., high level of CO) before it affects the performance of the other fuel cells.
U.S. patent application Ser. No. 08/998,133, filed Dec. 23, 1997, by the same applicant as the present application, discloses a fuel cell operating method in which a substantially fuel-free liquid (which may contain oxygen) is periodically introduced into the fuel stream in order to cause a fuel starved condition.